On what terms?
If you're asking how they track people just visiting their site, it's through the computer's IP address. Every computer has a distinct one, and if you do a quick search you can find your own, and also view the location (city) and internet provider for that computer.
If you're asking on terms of people with user accounts, it's (obviously) through their account. Most sites keep a history of what you look at and what you buy so that they can try and provide you with things you may like so that you will buy more.
Now if you're asking how they can track you outside of their site, that would be illegal and online stores are definitely not allowed to do that.
There are bunch of shopping sites in India. I have purchased so many things online. Report found that 60 percent of online users in India visited a Retail site.
Online auction sites
There are two types of websites that offer users free brochure website templates. Some websites require users to download software that contain brochure templates; but there are also sites like printingforless.com allow users to create the templates online.
You can use Paypal to send eChecks. It's also called an electronic funds transfer, because you withdraw money directly from your bank account. Paypal is popular with eBay users. However, most major online shopping sites do not accept Paypal payments.
Most online dating sites are not exclusive when it comes to the sexual orientation of their users, but there are a number of sites dedicated entirely to the LGBT community. PinkCupid is a lesbian dating site.
From experienced users, experimenting, and online sites such as erowid and wikianswers
There are a variety of online sites that enable users to post and share their photo albums. Examples of such sites include Flickr, Shutterfly, and Snapfish.
With online shopping growing in popularity recently there are many ways to save money online. One option is buying stuff used rather than new. Penny auction sites have also grown in popularity, allowing users to have a chance to buy items at a 90% or more discount.
On my personal list would be the Heaphy Track, the Tongariro Alpine Crossing, The Route Burn Track, and the Milford Track. You'll easily find web sites for these, with users comments.
The online magazine Adore provides a blog for users to read, shopping, and the option of purchasing a home delivered version of the magazine. Users can also read articles on the website that have been published in the magazine.
There are a number of online comparison sites that allow users to compare the price of certain auto insurance online, to find the cheapest available. Examples of these sites include ComparetheMarket, GoCompare and MoneySupermarket.
Blogo is a service that allows users to instantly publish text, images, and videos to online blogging sites. With support for many sites, Blogo allows users to update different blogs from the comfort of their desktop.